Method of fixing carbon dioxide

ABSTRACT

A method and system of fixing carbon dioxide is provided. After metal ion components are extracted from, e.g., natural mineral or steel slag through acid treatment, carbon dioxide is injected to fix carbon dioxide by carbonating the same. Since the procedure of pH adjustment is unnecessary, the reaction is carried out effectively and a continuous process is enabled. Accordingly, the disclosed method of fixing carbon dioxide enables effective removal of carbon dioxide produced from the steelmaking industry, thereby significantly reducing greenhouse gas emission and allowing recycling of the discarded steel slag.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Korean PatentApplication No. 10-2010-0120925, filed on Nov. 30, 2010, in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein by reference in its entirety.

BACKGROUND

(a) Technical Field

The present invention relates to a method and system of fixing carbondioxide. More particularly, the disclosure relates to a method forfixing carbon dioxide utilizing natural mineral or steel slag producedfrom ironworks, thereby reducing emission of carbon dioxide into theatmosphere.

(b) Background Art

Carbon dioxide emission is increasing rapidly due to the increasedfossil fuel consumption and is recognized as the main cause of globalwarming. Thus, many countries in the world have been making efforts andtightening regulations to reduce carbon dioxide emissions.

The reduction in carbon dioxide emissions can be mediated by reducingthe use of fossil fuel itself or by a method which separates, collectsand fixes the carbon dioxide produced thereof. For the latter,utilization of the separated and collected carbon dioxide as a sourcefor methanol synthesis, or fixing the separated and collected carbondioxide by dumping it into the ocean or using carbonate minerals havebeen studied in the past.

For example, one method fixes carbon dioxide using carbonate minerals.In this method, alkaline substances (CaO, MgO, K₂O, Na₂O, etc.) existingin minerals are reacted with carbon dioxide to produce carbonates(CaCO₃, MgCO₃, Na₂CO₃, K₂CO₃, etc.) to fix the carbon dioxide emittedfrom plants.

Slags produced from the steelmaking process include molten ironpretreatment slag, converter slag, stainless steel slag, electricfurnace slag, or the like. These steel slags are mostly buried, exceptfor limited utilization as cement or aggregates for road or buildingconstruction. Various methods for utilizing the discarded steel slaghave been proposed, as it becomes difficult to find landfills fordisposal.

Korean Patent Application Publication No. 2002-0050429, entitled“Pretreatment Method Of Steel Slag By Using Carbon Dioxide,” proposes amethod of fixing carbon dioxide on the surface of steel slag for use inharbor construction or artificial fish banks for fish. In anotherattempt to dispose of carbon dioxide using slag, Korean PatentApplication Publication No. 2006-0023206 entitled, “A Method for Fixingof Carbon Dioxide,” proposes fixing carbon dioxide on the surface ofslag containing a certain level of water by reacting it with carbondioxide. However, the methods described in the above two patents requiretoo long a reaction time since the efficiency of the reaction betweencarbon dioxide and slag is very low and, thus, are economicallyimpractical.

Korean Patent No. 0891551 (Application No. 2008-0025573), entitled “Solidification Method of Carbon Dioxide By Mineral Carbonation Of SlagGenerated In An Iron Industry, Capable Of Improving ReactionEfficiency,” proposes a method of solidifying carbon dioxide bycarbonating alkaline components extracted from steel slag with gaseouscarbon dioxide through a pressurized hydrothermal reaction or anormal-pressure hydrothermal reaction. However, the hydrothermalreaction consumes a lot of energy, and there is no clear descriptionabout how the carbon dioxide is fixed following the carbonation of thealkaline components.

Korean Patent No. 0801542 (Application No. 2006-0105753) entitled,“Method For Converting Talc For Mineral Carbonation By Removing WaterMolecules And Hydroxyl Groups, And A Method For Mineral Carbonation OfCarbon Dioxide Using Talc Obtained Thereby,” proposes a method of fixingcarbon dioxide using the natural mineral talc. However, since itrequires a particle size of 125 μm or smaller, excessive energy isconsumed for the pulverization of the mineral. Further, after thealkaline components are treated with a weakly acid solvent such asacetic acid for extraction, when pH is increased to fix the carbondioxide through carbonation, some metal ions (e.g., Ca²⁺) react withhydroxide ions (OH) to form milky lime (Ca(OH)₂), resulting insuspension. Thus, an additional precipitation or filtration procedure isrequired. Since the precipitated carbonate suspends in the solution, theseparation is difficult and it is not easy to establish an industriallyapplicable continuous process.

SUMMARY

The present invention relates to a method capable of overcoming theproblems present in the conventional methods, effectively fixing carbondioxide and being applicable to actual processes, whereby carbon dioxidecan be fixed via a liquid-phase reaction, and provide improved reactionefficiency, at a normal temperature under normal pressure, thus reducingenergy consumption. Additionally, in the present invention, theprocedure of pH adjustment for carbon dioxide fixation is unnecessary,thus preventing the formation of suspending milky lime and allowing acontinuous process.

In one embodiment of the present invention, a method of fixing carbondioxide includes (a) treating natural mineral or steel slag with an acidto extract metal ion components; (b) injecting carbon dioxide to anextraction solution containing the metal ion components obtained in step(a) to carbonate the same; (c) transferring a solution in whichcarbonates resulting from step (b) are dissolved to a dissolvedcarbonate storage tank and storing the same; and (d) transferring thestored dissolved carbonates to a carbonate separation/purification tankand adjusting pH to about 7 or above to separate the carbonates.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will now be described in detail with reference to certainexemplary embodiments thereof illustrated in the accompanying drawingswhich are given hereinbelow by way of illustration only, and thus arenot limitative of the disclosure, and wherein:

FIG. 1 shows an existing process of fixing carbon dioxide using slag;and

FIG. 2 shows a method of fixing carbon dioxide using natural mineral orsteel slag according to an exemplary embodiment of the presentinvention.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of thedisclosure. The specific design features of the disclosure as disclosedherein, including, for example, specific dimensions, orientations,locations and shapes, will be determined in part by the particularintended application and use environment.

DETAILED DESCRIPTION

Hereinafter, reference will now be made in detail to various embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings and described below. While the disclosure will bedescribed in conjunction with exemplary embodiments, it will beunderstood that the present description is not intended to limit thedisclosure to those exemplary embodiments. On the contrary, thedisclosure is intended to cover not only the exemplary embodiments, butalso various alternatives, modifications, equivalents and otherembodiments, which may be included within the spirit and scope of thedisclosure as defined by the appended claims.

The present invention provides a method of fixing carbon dioxide which(a) treats natural mineral or steel slag with an acid to extract metalion components; (b) injects carbon dioxide into an extraction solutioncontaining the metal ion components obtained in step (a) to carbonatethe same; (c) transfers a solution in which carbonates resulting fromstep (b) are dissolved to a dissolved carbonate storage tank and storingthe same; and (d) transfers the stored dissolved carbonates to acarbonate separation/purification tank and adjusts the pH to about 7 orabove to separate the carbonates.

In step (a), the natural mineral is not particularly limited. Forexample, peridotite, basalt, talc, serpentinite, wollastonite, etc.containing a large amount of calcium oxide and magnesium oxide may beused. The contents of calcium oxide and magnesium oxide in serpentiniteand wollastonite among them are shown as an example in Table 1.

And, in step (a), the steel slag may be slag, electric furnace slag orconverter slag produced, e.g., from the ironworks during blast furnace,converter or oxygen blowing processes. The steel slag is mostly utilizedas cement or aggregates for road or building construction. The chemicalcomposition of various steel slags is also shown in Table 1.

TABLE 1 CaO (wt %) MgO (wt %) Natural mineral Serpentinite 0 40Wollastonite 48 0 Steel slag Blast furnace slag 41 10 Converter slag 462 Electric furnace slag 20 5 Ladle furnace slag 54 10

Since the natural mineral and the steel slag contain a large amount ofmetal oxides including calcium oxide and magnesium oxide, they exhibithigh basicity and, upon acid treatment, the metal components of themetal oxides are released to the solution, thus exhibiting alkalineproperties. The metal ion components are mostly Ca²⁺ and Mg²⁺, and theremainder may be K⁺, Na⁺, etc.

Specifically, the acid treatment may be performed at pH 3-5. For this,an acidic substance such as acetic acid, sodium acetate, hydrochloricacid, etc. may be used at adequate concentration. Thus, upon the acidtreatment, the solution containing the metal ions, e.g., Ca²⁺, Mg²⁺,etc., has an acidic pH.

According to the existing method, the pH of the extraction solution isadjusted to about 12 and then carbon dioxide is injected to formcarbonate precipitate, for fixing by carbonation. During this procedure,some of the dissolved metal ions (e.g., Ca²⁺, Mg²⁺, etc.) react withhydroxide ions (OH³¹ ) to form milky lime, thus resulting in suspension.Unless the suspending solid is effectively separated, it becomesdifficult to fix carbon dioxide through a continuous process. As aresult, treatment of carbon dioxide in large scale is impossible.

To resolve the limitation of the existing method, the present inventionomits the procedure of adjusting the pH to about 12. By omitting the pHadjustment procedure, the formation of milky lime and consequently theformation of suspending solids can be prevented. Thus, a continuousprocess is enabled and treatment of carbon dioxide in large scale ispossible. According to the present invention, the procedure of adjustingpH to a predetermined alkaline range (e.g., pH 7 or above) forseparation of the carbonates is assigned after a CO2 reaction with metalion components is completed. Since the metal ions have participated inreactions to form the carbonates, the possibility of forming suspendingsolids through reaction with hydroxide ions is very low. The solution inwhich the carbonates resulting from the carbon dioxide fixing (step (b))are dissolved are transferred to a dissolved carbonate storage tank andstored there. Then, after the stored dissolved carbonates aretransferred to a carbonate separation/purification tank, the pH isadjusted to 7 or above to separate the carbonates. Consequently, theprocedures of carbonation and carbonate separation can be performedcontinuously.

In the present invention, the carbonation in step (b) may be performedat normal temperature under normal pressure. Here, the normaltemperature and the normal pressure mean that no additional heat orpressure needs to be applied. For example, the temperature may bebetween about 0-40° C. (32-104° F.), and preferably between about 10-25°C. (50-77° F.), and the pressure may be between about 0.1-5 atm, andpreferably between about 0.5-2 atm. In accordance with the presentinvention, the carbonation can be completed within about 5 minutes evenwithout the energy-consuming hydrothermal reaction. That is to say, thecarbonation reaction of step (b) can be completed within about 5 minuteswhen the injection rate of carbon dioxide is, e.g., 2 L/min Since therelease of the metal ion components in step (a) can be accomplishedwithin about 2 hours, the overall process can be completed in about 2hours, making it a very economical process. Thus, carbon dioxide can beeffectively fixed using natural mineral or steel slag, for example,slag, electric furnace slag or converter slag produced from theironworks during blast furnace, converter or oxygen blowing processes,thereby remarkably reducing the emission of the greenhouse gas andallowing the utilization of the previously discarded steel slag.

The carbon dioxide gas treated by the method according to the presentinvention may be any carbon dioxide gas, including those produced as aby-product of industrial furnaces such as blast furnace, lime burningfurnace, coking furnace, etc., sintering or hot-rolling processes, powergeneration, waste heat boilers, or the like. Accordingly, if a facilitycapable of fixing carbon dioxide generated in the steelmaking process isequipped at the ironworks, the steel slag produced during the processmay be utilized to fix the global warming-causing gas on site. Throughthis, the environmental regulation can be met and the by-products suchas metal oxides can be utilized as a new source of revenue, rather thanwastes.

EXAMPLE

The example and experiment will now be described. The following exampleand experiment are for illustrative purposes only and not intended tolimit the scope of this disclosure.

Example

Metal ion components were extracted from steel slag with a particle sizeof 1-5 mm using 10 vol % acetic acid at pH 3-5. The weight ratio of theextraction solvent to the slag was 10:1, and stirring rate was 150 rpm.Change in calcium ion concentration with extraction time is shown inTable 2.

TABLE 2 Extraction time (hr) 1 2 3 Calcium ion concentration (mg/L)26,000 38,000 38,000

As seen from Table 2, the extraction of the metal ions by the acidtreatment was completed in about 2 hours.

Carbonation (carbon dioxide fixing) was carried out by injecting carbondioxide to the resulting extraction solution. Carbonation was performedat 25° C. (77° F.) and 1 atm with the flow rate of carbon dioxidemaintained at 2 L/min Change in calcium ion concentration and pH withcarbonation time is shown in Table 3.

TABLE 3 Carbonation time (min) 1 1.5 2 2.5 3 3.5 4 4.5 Calcium ion con-38,000 22,000 18,000 16,000 13,500 13,200 13,000 13,000 centration(mg/L) pH 3.71 3.90 3.98 4.02 4.06 4.10 4.14 4.22

As seen from Table 3, carbonation was completed within about 4.5 minutesfollowing the injection of carbon dioxide.

Conversion ratio of calcium ions and purity of precipitating calciumcarbonate are summarized in Table 4.

TABLE 4 Calcium ion Conversion ratio Purity of concentration (mg/L) ofdissolved precipitating Before After calcium carbonate calciumcarbonation (A) carbonation (B) ((A − B)/A × 100, %) carbonate (%)38,000 13,000 65.8 99

As seen from Table 4, both the purity and yield of precipitating calciumcarbonate could be improved. Thus, the method proposed in the presentinvention whereby carbon dioxide is fixed by carbonation without pHadjustment, enables preparation of high-purity carbonate in about 2hours. Further, since a continuous process is enabled, treatment ofcarbon dioxide in large scale is possible.

Advantageously, the method of fixing carbon dioxide according to thepresent invention, whereby metal ions are extracted through acidtreatment of natural mineral or steel slag, is much more effective thanthe existing solid-gas reaction. Since the reaction is carried out atnormal temperature under normal pressure without requiring anenergy-intensive procedure such as hydrothermal synthesis, energyconsumption can be reduced. Further, since the pH adjustment for fixingcarbon dioxide is omitted, formation of suspending milky lime can beprevented and a continuous process is enabled.

Furthermore, since the method of fixing carbon dioxide according to thepresent invention allows reduction of carbon dioxide emission usingsteel slag which is currently used only for valueless applications, itcan be an effective measure to cope with the greenhouse gas reductionrequirement imposed on steelmaking companies while allowing recycling ofthe discarded steel slag as carbonate.

The illustrative embodiment of the present invention may also beembodied as a system for fixing carbon dioxide. In the system, a firstdevice may be configured to treat slag with an acid to extract metal ioncomponents. An extraction solution containing the extracted metal ioncomponents may be injected with carbon dioxide by a second device, e.g.,an injector, to carbonate the extraction solution. A solution in whichthe carbonates resulting from the injection of carbon dioxide aredissolved may then be transferred to and received by a first tank whichis configured to store the resulting carbonates. The stored dissolvedcarbonates are then transferred to and received by a second storage tankwhich is configured to adjust the pH of the carbonates to apredetermined range (e.g., 7 or above) to separate the carbonates.

The present invention has been described in detail with reference tospecific embodiments thereof. However, it will be appreciated by thoseskilled in the art that various changes and modifications may be made inthese embodiments without departing from the principles and spirit ofthe disclosure, the scope of which is defined in the appended claims andtheir equivalents.

1-13. (canceled)
 14. A system for fixing carbon dioxide comprising: afirst device configured to treat slag with an acid to extract metal ioncomponents; a second device configured to inject carbon dioxide into anextraction solution containing the extracted metal ion components tocarbonate the same; a first tank configured to receive a solution inwhich the carbonates resulting from the injection of carbon dioxide aredissolved and to store the resulting carbonates; and a second tankconfigured to receive the stored dissolved carbonates and adjust the pHto a predetermined range to separate the carbonates.